The present invention relates to an induction electrical apparatus and, more particularly, to a gas-insulated type stationary induction electrical apparatus.
A conventional induction electrical apparatus, such as that disclosed in Japanese Patent Examined Publication No. 61-27888, is provided with a diffuser and a spray whereby a liquid serving as a cooling and insulating medium is evenly dispersed onto such members of the apparatus as the iron core and the windings. The "electromagnetic induction equipment" disclosed in Japanese Patent Unexamined Publication No. 58-158906 is provided with a vessel with insulating characteristics in which parts of the iron core and the windings are dipped in a condensable cooling liquid and which covers the outer and lower sides of the windings, various cavity portions of the apparatus being filled with a casting resin. With the arrangements of both these two types of induction apparatus, however, it is difficult to ensure an adequate cooling effect.
Japanese Patent Unexamined Publication No. 61-111513 discloses a "vaporization-cooled induction electrical apparatus" provided with a first closed vessel containing SF.sub.6 gas under a high pressure and a coolant, and a second closed vessel accommodating the first closed vessel, the gap between these vessels being filled with a compressed gas under an intermediate pressure, the apparatus thus being a stationary induction electrical apparatus having a double closed-vessel structure.
In the above-described gas-insulated induction electrical apparatus, SF.sub.6 gas having a high level of insulating ability is used as an insulating medium, and the gas is also used as a cooling medium. However, when the induction electrical apparatus has a large capacity, since a large amount of heat is generated by the apparatus, the heat transfer rate achievable with SF.sub.6 gas often proves to be insufficient.
The cooling effect provided also falls short of adequacy when the apparatus concerned is an induction electrical apparatus provided both with a vaporization cooling system in which a coolant formed of a liquid having a low boiling point is dispersed onto the iron core, etc. so that the temperature is caused to drop utilizing the heat of vaporization, and with a SF.sub.6 gas insulating system, and when the apparatus has a large capacity and can generate a large amount of heat. Particularly when the iron core used has a complicated structure, it is impossible to prevent localized overheating of the iron core. When such an iron core undergoes localized overheating, and when the iron core is formed using silicon steel sheets as the material, Si contained in the steel chemically reacts with SF.sub.6 gas to generate SiF.sub.4 , which may promote corrosion of the silicon steel sheets. This chemical reaction occurs in the following manner: EQU 2SF.sub.6 +6H.sub.2 O.fwdarw.2SO.sub.2 +12HF+O.sub. EQU SF6+O.sub.2 .revreaction.SO.sub.2 +3F.sub.2 EQU Si+2F.sub.2 .fwdarw.SiF.sub.4
The "localized overheating" will be described further and in detail. As shown in FIGS. 4A, 4B and FIG. 5, an iron core in general has a structure including a core main leg 41, a core upper yoke 42, a core lower yoke 43, and a core side leg 44, etc. The illustrated structure of the iron core is formed by laminating a plurality of silicon steel sheets 45. If fine gaps 46 (see FIG. 5) are formed in a junction A of the steel plates 45, when magnetic flux 47 flowing through the iron core is passing through the junction A, the flux encounters a large magnetic resistance at the gaps 46 in the junction A, and it is thus caused to transfer to an adjacent silicon steel sheet 45. As a result, the flux density increases at the location where this transfer occurs, and this leads to localized overheating.
FIG. 6 shows a graph in which the temperature of the iron core is used as the parameter, and in which the axis of ordinate represents the amount of gases (SO.sub.2, SiF.sub.4, etc.) generated, while the axis of abscissa represents the amount of moisture within the associated first closed vessel. As will be clearly understood from this graph, when the iron core is locally overheated, silicon (Si) contained in the iron core may chemically react with a small amount of water (H.sub.2 O), thereby leading to the generation of the gas which may promote corrosion of the iron core.